There are numerous types of filter in existence. The so-called GREENFIELD filter, which consists of wires preformed into zig-zags and joined together by a cover, is particularly well known. These wires are arranged to form a cone whose tip is the cover, said zig-zags forming the envelope of the cone. The ends of the wires are provided with anchoring hooks by which the filter can grip the inner wall of the vessel.
The placing of such a filter is effected from an insertion sheath inside which the filter is inserted, the elastic wires being kept approximately in alignment with the longitudinal axis of the sheath. After the distal end of the insertion sheath has been positioned at the location where the filter is to be placed in the vessel, said filter is released with the aid of a thrust component which is a rod capable of being actuated from outside the insertion sheath, and which, bearing on the filter, makes it possible to move the latter while at the same time keeping the insertion sheath steady. Once released in the vessel, the elastic wires develop into a cone inside the vessel, the end hooks on the wires gripping the inner wall of the vessel.
Other filters based on the same working principle have been proposed since this GREENFIELD filter, one having a conical general shape with several lugs joined together at their heads, forming an ogival point and diverging towards their other end. This filter is in a single piece formed from a thin sheet of a material of appropriate elasticity. The lugs in question are therefore flat strips of small thickness. Such lugs are tapered at their free end. For positioning inside the vessel, this filter is placed in an insertion tube. A flexible metal guide is first introduced into the vein; then, using this metal guide, a first, relatively thin insertion tube is introduced which is used as a guide tube for the passage of the insertion tube containing the filter.
Thus, as well as the flexible metal guide, the placing of such a filter requires the use of two insertion tubes.
The Applicant has found various disadvantages to be associated with the devices currently in use. On the one hand, the first insertion tube, through which the second insertion tube containing the filter has to travel, must have a relatively large overall diameter because the second insertion tube can have an internal diameter of 3.6 mm. Thus the first insertion tube may possibly have an external diameter of the order of 5 mm. These tubes are made of a flexible plastic material which must nevertheless be sufficiently rigid to be able to advance inside the vessel while at the same time delimiting an interior space capable of accommodating the second insertion tube. Depending on the sinuosity of the vessels, especially in the case of the left femoral vein, the first insertion tube sometimes becomes prohibitively bent, preventing the second insertion tube from traveling through it normally.
The object which the Applicant set out to achieve is to propose a kit, composed of a filter and a device for placing it, which overcomes the observed disadvantages in that it is of simple design and does not require insertion tubes of large diameter.
This object is perfectly achieved by the kit of the invention, which is composed of a filter, a flexible metal guide, a push-rod and an insertion tube. The filter is formed by elastic lugs extending approximately in the shape of a conical corolla extending from a distal head, which has a small orifice of diameter D1 in its tip; the metal guide has an external diameter D2; the push-rod is a hollow tube of internal diameter D3; the insertion tube, which is made of plastic, is fitted at its distal end with a rigid cylindrical cap of internal diameter D4. The cap has an annular internal set-back at its proximal end, which is capable of acting as a stop for the ends of the lugs of the filter. This annular set-back also serves to center the push-rod. The respective dimensions of the different components mentioned above are such that D4 is greater than D3&gt;D1&gt;D2, so that, in the position of insertion of the filter into the vessel, the tip of the filter is facing forwards, the ends of its lugs being folded back inside the cap, and the distal end of the push-rod bearing against the inner part of said tip.
The placing of the filter by means of the kit of the invention is effected by first introducing the flexible metal guide into the vessel and then threading the filter itself, which is positioned at the end of the insertion tube, over said guide, the lugs being folded back and placed inside the cap which terminates said insertion tube. Thus it is seen that when the assembly consisting of the filter, the insertion tube and the push-rod travels into the vessel, the filter itself is not enclosed in an insertion tube but is outside it. Only the free ends of the lugs of the filter are locked in the cap between the inner face of said cap and the outer surface of the push-rod.
It is seen that, under these conditions, the insertion tube can have a smaller diameter, for example of the order of 3 mm.
By keeping the flexible metal guide inside the vessel during the introduction of the other components of the kit, it is possible to avoid any risk that the filter will become stuck in the sinuosity of the vessel. It may also be pointed out that the filter, held solely by the free end of its lugs, has a degree of flexibility which facilitates the passage of this assembly through the particularly sinuous parts of the vessel. It is of course important for said lugs to be adequately held between the cap and the push-rod. For this purpose, the cap has a length of at least 10 mm for a lug length of between 40 and 60 mm.
The filter preferably has six lugs whose width at the proximal end is approximately equal to or less than half the internal diameter D4 of the cap. Thus, in the folded-back position, the six lugs are contiguous and form a regular hexagon approximately inscribed in the cap.
In the case where the ends of the lugs have means of anchoring to the inner wall of the vessel, these means preferably consist of two hooks pointing in opposite directions. More precisely, each hook is obtained by the cutting-out of a triangular portion of the strip forming the distal end of the lug; moreover, it has an elbow-shaped configuration relative to the plane of said lug. This particular arrangement reduces the height of said hook outwardly of the lug and consequently reduces the space which is required between the push-rod and the cap for accommodating each lug when the filter is positioned. However, this configuration in no way reduces the good grip of the filter on the wall of the vessel.
To avoid any false maneuvers in the placing of the filter, the insertion tube is terminated at its distal end by a shoulder which is intended to remain outside the patient's body; furthermore, the kit has a hollow protective cover which can be firmly fixed to said shoulder, for example by screwing, and which covers the proximal end of the push-rod, the latter extending beyond the insertion tube when said cover is in the fixed position. Thus the operator cannot move the push-rod accidentally; to move it, he has to release the protective cover and remove it in order to have access to the proximal end of the push-rod.